It is conventional to make innertubes in several ways. One method is to mold the tube under pressure in a heated mold. The resulting tube is smooth and has a circular cross section. Press molding of tubes is the best method but is much too expensive for the prices at which bicycle tubes commonly sell.
Another way of making innertubes involves extruding openend tubes which are either vulcanized on circular mandrels, or as disclosed U.S. Pat. No. 545,118, inside a circular or eliptical casing, after which the valve stems were installed and the ends of the tubes spliced to form a continuous inner tube. German Auslegeschrift 2,049,453 shows a minor variation in which a valve hole is punched and the ends of the length of tube-like extrudate are spliced after which the continuous tube is folded in half and laid on a surface for open vulcanization. The use of mandrels or casings to support the flat length of tube is too troublesome and expensive. The open-vulcanization of folded tubes produces innertubes having sharply and permanently-fixed edge folds and end folds which are objectionable to purchasers accustomed to the known quality of molded tubes and, moreover, are the loci of flex-, fatique- or oxidative-cracking in service.